utility: given a schema document, unroll the $refs within

[karenetheridge]

Replace each $ref with its definition and return the resulting schema. This could be useful for code generation or other tools that require very simplistic schema definitions.

This will break down if there are circular or recursive references, so make sure to use a max depth counter and abort if it's hit.

This will probably work as-is for OpenAPI documents too, since ref resolution works the same way there.